This invention relates generally to fiber optic couplers, and, more particularly, to a coupler which is capable of connecting two optical fibers together in a manner which is both optically efficient and substantially immune to various types of misalignment.
In recent years optical fibers have proved to be highly successful in the transmission of light in optical communication systems. Such systems can be broadly classified as either multimode or single mode, according to the type of optical fiber employed. Generally, it has been determined that the single mode system offers advantages such as higher transmission capacity and appropriate compatibility with integrated optical circuitry over multimode systems.
Unfortunately, great difficulty is generally incurred when coupling or connecting adjacent optical fibers together under field conditions. Such difficulty results from the misalignment of the minute diameter (a few microns for a monomode fiber) light guiding cores of adjacent fibers. Consequently, numerous companies have been involved in the development and/or manufacture of optical couplers or connectors which attempt to solve these alignment problems.
Basically, there are two quite separate design criteria associated with the coupling of optical fibers: (1) the design of an optical system which maximizes the tolerance of a connector or coupler to mechanical misalignment (i.e. which minimizes the insertion loss increment due to mechanical misalignments), and (2) the design of a mechanical connector or coupler which minimizes misalignment. Stated more succinctly, the objectives in designing a connector or coupler for field use, in particular, are first to provide for good mechanical alignment between the fibers and then to make sure that there are minimal losses associated with the residual misalignments. In general, there are three types of misalignments: (1) lateral; (2) axial or longitudinal; and (3) angular.
To date, the dominant optical designs have been either the butted fibers as described by J. E. Midwinter in Optical Fibers for Transmission, J. Wiley and Sons, New York, 1979 or the collimated beam coupler as described by O. D. D. Soares in an article entitled "Holographic Coupler for Fiber Optics", Optical Engineering, September/October 1981, Vol. 20, No. 5.
The usual approach to fiber coupling is the butted end coupler which leads to the interconnection of the fibers by careful preparation of the fiber end faces and by using micropositioning devices to bring the fiber into angular, lateral and axial alignment. Unfortunately, the requirement for stringent alignment results in connectors which are primarily capable of use in a laboratory and which are unlikely to be reproducible. In addition, since the butted fiber arrangement requires precise mechanical alignment and is particularly sensitive to lateral displacement, this type of connector is almost completely useless in field applications. For example, a lateral displacement equal to the fiber core diameter can result in complete loss of transmitted signal through the connector or coupler.
The collimated beam connector design, although substantially successful in avoiding sensitivity to lateral displacement, is generally very sensitive to angular misalignment. For example, if the angular misalignment of the fiber cores equals or exceeds the ratio of the optical fiber core diameter to the focal point of the collimating lens, then the signal is totally lost at the coupler or connector. Consequently, the collimated beam connector is less than acceptable in field applications.
It is therefore clear, that it is essential to provide a connector or coupler for optical fibers which is substantially immune to various types of misalignment problems and which can be reliably and efficiently used in the field.